The callback of showDatePickerDialog and showTimePickerDialog always return with the cancel property set to false. It doesn't matter if you press the back button or press outside the modal dialog, cancel gets never true.
Furthermore, if you click on the Set/Ok/Done/Ready button, the callback is called twice.
Note that in the picker dialog, we have only a single button "Set", so no Cancel button. This is fine as you should be able to cancel dialog with the back button. Clicking the back button fires wrongly a success (cancel=false) event.
Problem does not occur on older Android devices, like Android 2.3.
See https://github.com/markruys/titanium-test-picker for a working project to reproduce.